The present invention relates generally to an exposure apparatus, and more particularly to a cleaning of a mask (or a reticle) used for an exposure apparatus. The present invention is suitable, for example, for an exposure apparatus (referred to as an “EUV exposure apparatus” hereinafter) that uses an extreme ultraviolet light (“EUV light”) as a light source for exposure.
A projection exposure apparatus has been conventionally used to transfer a mask pattern via a projection optical system onto a wafer etc, and the exposure apparatus with a high resolution has recently been increasingly demanded. As one means for implementing the high resolution, use of an exposure light having a shorter wavelength is promoted, and an EUV exposure apparatus using an EUV light with a wavelength of 10 nm to 20 nm shorter than that of the ultraviolet light has recently been developed.
Since a light absorptance in a material remarkably increases in a wavelength range of the EUV light, the EUV exposure apparatus generally uses a catoptric optical system that does not use a refractive member. Therefore, a pellicle used for a dioptric optical system cannot be provided to the mask, and a mask pattern surface is exposed. Here, the “pellicle” is a thin film having a high transmittance that covers the mask pattern surface and prevents adhesions of particles or dust onto the pattern surface. The particles originate from a driving part that drives the mask and residual gas. The particles that adhere to the mask pattern surface cause patterning defects. Therefore, it is necessary to remove the particles from the mask pattern.
One proposal to reduce particle adhesions to the mask uses a thermophoretic and/or electrostatic force. See, for example, D. J. Rader, D. E. Dedrick, E. W. Beyer, A. H. Leung and L. E. Klebanoff, “Verification studies of thermophoretic protection of EUV masks,” Emerging Lithographic Technologies VI, SPIE Proceedings Vol. 4688 (2002), and R. Moors, G.-J. Heerens, “Electorostatic mask protection for extreme ultraviolet lithography”, Journal of Vacuum Science & Technology B, Vol. 20, No. 1, pp. 316-320 (2002). This method can reduces particle adhesions but cannot completely remove the particles. Then, a method that removes the particles by using a chemical cleaning outside the exposure apparatus and a method that removes the particles by irradiating pulse laser outside the exposure apparatus have been proposed. See, for example, Japanese Patent Applications, Publication Nos. 1-12526 and 2-86128.
Another prior art are Scime et al., Extreme-Ultraviolet Polarization and filtering with gold transmission gratings, Applied Optics, Vol. 34, No. 4, 1 Feb. 1995, Laser-assisted removal of particles on silicon wafers, pp 3838-3842 and “Cleaning of silicon wafer surface using excimer laser”, P 80-83.
However, conventional particles removing methods have the following problems. The chemical cleaning cannot be executed inside the exposure apparatus, and the particles re-adhere while conveying the cleaned mask from outside the exposure apparatus to inside the exposure apparatus. Moreover, the irradiating method of pulse laser disclosed in Japanese Patent Applications, Publication Nos. 1-12526 and 2-86128 cannot efficiently remove the particles because a width of the mask pattern for EUV exposure apparatus is below the wavelength of pulse laser and light does not reach a pattern groove depending on a polarization direction of light. When light is incident upon the pattern having a width below the wavelength, a light intensity of light that reaches an inner part of the pattern groove has a polarization dependency as described in Scime et al., Extreme-Ultraviolet Polarization and filtering with gold transmission gratings, Applied Optics, Vol. 34, No. 4, 1 Feb. 1995. For example, light having electric field component perpendicular to the groove can reach the inner part of the groove but light having electric field parallel to the groove cannot reach the inner part of the groove.